The present invention relates to a printhead as well as a printing apparatus comprising the printhead, and in particular relates to a printhead, which can allow driving conditions to be equal in a plurality of printing elements connected to a common power supply, as well as a printing apparatus comprising the printhead.
As an information outputting apparatus in, for example, a word processor, a personal computer, a facsimile, etc., there is a printer which records information such as desired characters and images onto printing media in a sheet form such as a paper sheet and a film.
As a printing system of a printer, various systems are known, and in recent years, an ink-jet system has caught attention because non-contact printing onto printing media such as a paper sheet, etc. is feasible and colorization is easy and is very calm, etc. As its configuration, a serial printing system is being widely used, which is equipped with a printhead to discharge ink according to desired record information and performs printing while reciprocally scanning in a direction perpendicular to the feed direction of the printing media such as a paper sheet, etc., since it is inexpensive and can be miniaturized easily.
Among ink-jet systems, a bubble jet printing system is a system which heats and evaporates ink rapidly with a heating body (heater) so as to discharge ink droplets from an orifice with pressure of created bubble.
FIG. 4 is a circuit diagram showing an example of a heater driving circuit inside a printhead of an ink-jet printer which performs printing by a bubble jet storage method.
Heater elements R1 formed onto element substrate of the printhead and switching elements Q1 for switching currents to those heater elements are connected in series between a power supply VH and the ground, and control signals corresponding with storage information from the main body of the printer switch on or off any switching element to cause nozzles corresponding to the driven heater element to discharge ink thus forming an image.
In order to obtain a high quality image in a printer having a printhead which discharges ink by utilizing heat energy generated by such heater elements, it is necessary to keep the volume of ink droplets discharged to be stabilized constant. For that purpose, it is desirable that the heating value of the heater is kept constant.
Here, the heating value P in a heater converting electric energy into heat energy is expressed as:
P=(V2/R)txe2x80x83xe2x80x83(1)
Where
V: potential difference in the heater
R: resistivity of the heater
T: voltage application time.
As apparent from the equation (1), the heating value in a heater varies largely depending on heaters"" resistivity as well as voltages to be applied to heaters. Among them, the heaters"" resistivity has a variation around 20% due to heaters"" manufacturing process. As a method to suppress such a variation to affect heating value, methods described in Japanese Patent Laid-Open No. 7-76077 and Japanese Patent Laid-Open No. 10-95116 are known.
The method described in the former publication is the one in which resistivity of a dummy heater formed of the same material as the heater for ink discharge inside the printhead is measured, resistivity of the heater for ink discharge is calculated with this resistivity, and according to the calculated resistivity of the heater, pulse width of pulse signals to be applied to the heater is adjusted to optimize the heating value of heaters.
In addition, in the method described in the latter publication, on-resistance of switching elements such as MOS transistors, etc. to be directly connected to heaters suffer has a variation due to manufacture. Since the on-resistance of this MOS transistors is inserted between a power supply and the ground in series with heaters resistance, a voltage applied to the heater will be a power supply voltage divided by a ratio of the heater resistance to the on-resistance of the MOS transistor.
Therefore, a variation in the on-resistances of MOS transistors is equivalent to a change in the component V in the equation (1), influencing the heat values of the heaters. In order to suppress this influence, as in the methods of the above described publications, a method is used in which a dummy MOS transistor is formed inside a printhead, on-resistance of this MOS transistor is measured, a voltage V applied to a heater is calculated, and with that result, pulse width of pulse signals to be applied to the heater is adjusted so that the heating value of heaters is kept constant.
However, in the above described prior art embodiment, a variation of heaters resistance and on-resistance of MOS transistors connected to the heaters in series was taken into consideration as a factor which influences the heating values of heaters. Beside this, the followings are considered as factors which influence the heating values of heaters, but these points were not taken into consideration.
A variation as well as voltage change in an initial state of a power supply voltage supplying an electric power to heaters leads to a variation in voltage to be applied directly to heaters. In addition, a resistance component in a connector connecting the wiring and printhead to the main body of the printer is connected (in series) between heaters resistance and a power supply so as to give rise to a voltage drop due to these resistances, and therefore change in voltage to be applied to heaters.
Moreover, the on-resistance of the above described MOS transistors is not always constant but changes as a function of temperature changes and the gate drive voltage.
Conventionally, measures have been taken against these factors, but actually no effective measures have been provided. For example, it is practiced that against the power supply voltage changes, in order to reduce the voltage a variation at the time of shipping of products, the specification on the power supply voltage is made strict or the voltage in the vicinity of he head is attempted to be stabilized with a stabilizing circuit, but such arrangement will give rise to problems of cost increase of a whole printing apparatus due to power supply""s cost increase as well as increase in the number of components due to addition of additional circuits.
Wiring resistance or parasite resistance such as resistance in connectors is addressed by designing them to be sufficiently small in relation to the heaters resistance, but since the number of printing elements and heaters increase as the printing apparatus is made to provide higher quality images and rapid operations, currents flowing in wirings increase accordingly, giving rise to large voltage drop due to these parasite resistances, which has become a problem that cannot be ignored.
A first object of the present invention is to provide a printhead that can suppress influence of changes in power supply voltage and wiring resistance of a power supplying line, etc. and keep drive conditions equal on each printing element.
A second object of the present invention is to provide a printing apparatus comprising a printhead that can suppress influence of changes in power supply voltage and wiring resistance of a power supplying line, etc. and keep drive conditions equal on each printing element.
The above described first object is attained by a printhead of the present invention which is a printhead having a plurality of printing elements connected to a common power supply, wherein each printing element comprises:
a switching element connected to the above described printing element in series for controlling driving of the above described printing element with a voltages applied to a control terminal;
a constant voltage source using the above described power supply as a standard; and
a voltage control circuit for controlling the terminal-to-terminal potential difference of the printing element to be equal to a voltage of the above described constant voltage source when the above described printing element is driven.
In addition, the above described second object is attained by a printing apparatus of the present invention which is a printing apparatus for storage by a printhead having a plurality of printing elements connected to a common power supply, wherein each printing element comprises:
a switching element connected to the above described printing element in series for controlling driving of the above described printing element with a voltages applied to a control terminal;
a constant voltage source using the above described power supply as a standard; and
a voltage control circuit for controlling the terminal-to-terminal potential difference of the printing element to be equal to a voltage of the above described constant voltage source when the above described printing element is driven,
and the apparatus comprises drive control means for controlling driving signals to be supplied to each printing element so that energy amount consumed by each printing element is equal.
That is, the present invention provides a printhead having a plurality of printing elements connected to a common power supply comprising a switching element connected to the above described printing element in series for controlling driving of the above described printing element with a voltages applied to a control terminal; a constant voltage source using the above described power supply as a standard; and a voltage control circuit for controlling the terminal-to-terminal potential difference of the printing element to be equal to a voltage of the above described constant voltage source when the above described printing element is driven to each printing element, wherein when storage, driving signals to be supplied to each printing element is controlled so that energy amount consumed by each printing element is equal.
Such arrangement provides high image quality by keeping a consumed energy amount in each printing element, that is, an energy amount generated at the time of driving constant regardless of external conditions and environments of the printhead.
Accordingly, changes in power supply voltage and influence of wiring resistance and parasite resistance can be reduced and costs for a power supply apparatus and wiring can be reduced. In addition, since each printing element can be driven under constant conditions regardless of changes in characteristics of internal element due to temperature changes of a printhead, storage quality can be maintained.
Moreover, it will become unnecessary to apply a voltage including additional portion as a margin equivalent to voltage drop anticipated in wiring or connection portions to a printing element for driving as conventionally conducted, and the printing element can be driven under optimum conditions, so durability of the printhead will be improved.
The voltage control circuit preferably include a dummy printing element connected to a printing element in parallel and having the same characteristic as the printing element, a dummy switching element connected to the dummy printing element in series and having the same characteristic as a switching element, and a detecting element for feeding back the detection output to a control terminal of the dummy switching element so that terminal-to-terminal potential difference of the dummy printing element is equal to the voltage of above constant voltage source.
In this case, the detection output is preferably used as a power supply for a logic circuit connected to a control terminal of the switching element to which selection signals are inputted indicating whether or not the printing element should be driven.
In addition, a constant voltage source is preferably a voltage source utilizing a band gap voltage.
Moreover, a switching element is preferably a MOS transistor.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.